Mounting Outdoor A/C Units On A Roof

[TSJHD1]

I have a renovation which calls for mounting 2 outdoor units on top of a roof with a pitch of 1.5/12, that will have a TPO membrane. Neither the roofer nor I like the architect's design: 4- 4x4 posts penetrating the membrane to a minimum height of 8" from top of roof deck to top of post. The posts will be at the 4 corners of each unit, and will have a 2x8 header mounted on top of them, forming a square box, as seen in plan view. The inside of the box will have 4x4 steel angle mounted to it on all 4 sides, which the units will rest on. The detail shows copper flashing on the sides of the 4x4's.
AC Unit Mounting.jpgAC Unit Mounting.skp
The roofer pointed out that the upper parts of the 4x4's will eventually check, split, and leak, and the tops of the 2x8's have no protection from any kind of water intrusion at all.

The roofer is going to propose a different design along with his proposal, one that he will feel confident in standing behind, but I was also curious what you might suggest for this scenario.

Btw, structurally, the roof is vaulted, 2x10's 16" o.c, with spray-foam insulation. The 4x4's get sandwiched between a 2x10 on each side, thru-bolted w/ 1/2" bolts. The angle iron is also thru-bolted w/ 1/2" bolts. The units each weigh 316lbs, and are arranged in line with each other, meaning, if you were to draw a centerline through both units, that line would be parallel to the rafters.

There is no more than 8' of roof between the uppermost unit and the roof peak. So is there any reason why I couldn't just frame a level 3' square section of roof, and have the TPO simply conform to it, and then set the units directly onto it? Debris/snow buildup over time? More vibration/noise transfer?

And I suppose the copper flashings can be brought up and onto the tops of the 4x4's. But there has to be a simpler way of mounting the units to those posts.

TIA!

Tom

[newman]

Sounds like a recipe for desaster. Rather than 4x4's and 2x8's, why not just have a frame fabricated from steel tubing and powdercoated?
Makes for a strong and more compact design.

[tjbnwi1]

This what they make Uni Strut for. You could bolt that up with the stock fittings and be done in less than an hour each. I wouldn't penetrate the deck either, I would build it to layout to the solid members then Ledger or Head Lock it in place.

Look at some of the roof sealing products from this company to back up the membrane;

http://www.eternabond.com

Tom

[MarkMc]

I'd imagine that someone is expecting a warranty on the finished roof; whoever is expected to provide that will drive the detailing. If the warranty for the roofing doesn't cover the penetrations there's much potential for 'discussion' later should need arise. Personally I'd say 2" sched 40 galv pipe up which give a bit of flexibility on both ends and you should be able to readily get tpo boots and collars.

[NW Architect]

Nice, round, galvanized fence pipe for the legs and pipe boots for the roofing detail. Unistrut or angle structure for the frame

[ThingOfBeauty]

A third vote for round posts with factory TPO boots here.
Platform can be anything from there.
Do look into anchoring the AC units to the frame. Our summer thunderstorms can put quite a wind load on the units.
Might be a decent idea to build a larger plaform than strictly required, flexibility since condensers get bigger every few years.
HVAC guys can also provide sound isolation pads for a very reasonable price, seem like a no-brainer, though the last ones we did had none and you can't hear them at all.

[olllee]

The solar industry has many different of roof mounting boots ,collars and flashing kits for roof mounting all types of equipment . All designed for up lift. A lot of this stuff is pretty new with the big push for solar PV and solar thermal .
Several types let you mount and adjust to almost any angle , and with unistrut as has been said should be easy

[aptbldr]

Not familiar with TPO boots/collars.
Are these replaceable in Subject situation?

[ThingOfBeauty]

Here's an example of a TPO boot designed for installation at the time of the roof. That's what I was thinking of.

Apparently they also make ones designed for retrofit applications.

[MarkMc]

Here's the last ones we used. Had ~600 made up in the two sizes we needed. The big advantage for us was they were installed as a part of the new roof and thus fully warranted. That deck had 4" foam and orig. composite under TPO that we had to hit 3" framing on.